A standard type of screen is known having a frame formed by a pair of parallel longitudinal beams whose ends are bridged by transverse beams. A screen is spanned between these longitudinal and transverse beams and the entire assembly lies at a slight inclination to the horizontal, with the transverse beams horizontal and the longitudinal ones tipped. This frame is either supported on springs or elastomeric bodies so that it can move relative to a fixed support or it is hung by flexible links so that such motion is possible. In use material to be screen classified is dumped onto the upper end of the mesh, so that as the material moves down along the mesh the undersize or minus portion will fall through the mesh and the oversize or plus portion will remain on top of it.
The drive for such an apparatus is normally a simple electric motor that is flanged directly to one of the transverse beams and has a horizontal shaft extending parallel to the transverse beams and carrying an eccentric weight. As the shaft is rotated at high speed this weight therefore generates a throw which is transmitted via the transverse beam carrying the motor to the entire frame.
Since all of the energy is transmitted from the motor to the frame via the transverse beam carrying the motor, as described in German patent document 2,112,577 filed Mar. 16, 1971 by the instant inventor with G. Erlenstaedt, this transverse member must be designed to be enormously strong. In fact this transverse beam carrying the drive motor normally accounts for half of the weight of the screen machine. With a screen area of approximately 2 m.times.6 m the overall weight can be between 5 and 6 tons. Obviously such massive construction increases not only the material costs for making such a screen, but also requires that a very large drive unit be provided in order to impart the necessary motion to the shaker frame whose inertia is very large.